I looked up my area's average temperatures and found this: Daily temperatures rise above 85' on an average of only one day per year and fall below 23' on an average of only 2 nights per year. During the winter, the average daily high and low temperatures are 47' and 38', respectively. The summer months are equally mild, with an average high temperature of 68' and low of 53'. With this info, do I still need a complete air exchange once per minute? I've read both of your responses and now wonder if I have an exhaust fan in the upper north gable end set to come on at temperatures above 80', and have 4 automatic vents on the lower opposing side windows, as well as the screen door opened in the south side shouldn't that be sufficient airflow in my moderate climate? I'd like to stay away from the automatic roof vents because they seem to fail so quickly, much faster than the ones in the side windows. And, what temperature am I aiming for? Is about 80' good for my peppers and tomatoes and for my seedlings? I am still eating lettuce from last fall's outdoor sowing, so I don't expect to need much extra heat in the greenhouse in winter. (Sorry, I just love to brag about my winter lettuce!) Again, thank you for your advice. I just want to do this right the first time.

At 85 F. my top univent opens about 12 in. and the bottom vent opens about 8 inches (since it is cooler at the bottom. One thing to try is to put the pistons in a refrigerator for a few hours before installing. That fully retracts the piston so you can get maximum extension after installing. Passive ventilation can only help reduce inside temperatures somewhat. It will still be warmer inside than outside, sometimes by a considerable amount. Exhaust fans, shade cloth, misting, etc. are all necessary in my area once the outside temps. get above 85-90 F.

Thanks for posting this and the one of your greenhouse. It looks very nice. Please see my reply to your first post regarding the univent.

I've got one on a top opening window on a wall, it's not yet mounted properly so I can't comment on its full behaviour, but I suspect it will need some assistance staying shut - the spring isn't quite strong enough. If I still have problems when I've properly attached the two plates I'm going to add a weight and pulley to balance the weight of the window.